Pachycephalosaur head butting

Despite my interests in dinosaur behaviour I have rather managed to avoid the question of pachycephalosaurs so far and with a couple of nice photos on cue it seemed a good time to discuss this at least superficially. I don’t think this clade has actually even been mentioned here at any point so this is longer overdue.

Since I try to cover even the basics of archosaur palaeontology on here I should probably give a bit of background to these bone heads (as they are occasionally know – the literal translation of the name being thick headed reptiles). Pachycephalosaurs are a group of ornithischian dinosaurs closely allied to the ceratopsians (the horned dinosaurs) and with them make up the large and important clade the Marginocephalae. They were herbivorous bipeds that only spanned a relatively small range of sizes from small to medium (compared to many of their relatives) with the largest genus, Pachycephalosaurus, being up to around 5 m long.

Obviously their most prominent characteristic is the massively thickened skull roof and the occasional fringe of spines and knobs that run around the crown of the skull. What these were actually used for has long been contested with the most obvious suggestion being that these were used to fight with, either with each other or to attack other animals (like predators). Evidence has gone backwards and forwards over this with papers saying the head could not have absorbed impacts of fighting, or could have done, that they would clash heads or would not and would target flanks and that these were ornamental or not. In short, the only real consensus is that there is no real consensus as yet.

This may come as a surprise as despite the obvious controversial nature of many questions in palaeontology many are at least close to a consensus or the evidence has started to tip decisively but here this is not really the case. Part of the problem is likely to be the sparsity of material – pachycephalosaurs are not known from many good specimens at all (half a dozen are known from only skulls, partial skulls, or just the domes) and some aspects of their anatomy are thus not well understood. Combined with the relative lack of interest in this clade (since almost everyone seems to prefer theropods) it is perhaps less of a surprise.

The lack of material in Europe especially and the fact that the group is not half as well known as the ‘classics’ like tyrannosaurs and ceratopsians, and their relative small size means that they rarely make it into dinosaur halls outside North America so I was pleased to see two different displays of them in Japan – the first time I’d actually seen any. At the top we have a butting pair from Tokyo and below the front/side and back of a skull from Fukui (both images used with permission). I hope more research goes into this area as it is genuinely fascinating and covers various aspects of mechanics, ecology and behaviour that integrate well and of course the application of data and studies from living animals would be especially useful.

I’m really not sure what pachycephalosaurs were doing with those heads, but I find the “only for visual display” hypothesis a bit hard to swallow. I just don’t think that an animal walks around with 10 inches of solid bone in its skull just to show off to the ladies! I feel a structure such as a ceratopsian frill would be far more suited for that. They were using their heads actively, but for what? That is the question.

Of course, this is all just my own personal, irrevelent, idiosyncratic opinion, but I thought I’d mention it anyway.

Well I would *largely* agree with you, but the truth of course is that evolution throws up some incredible oddities from time to time. If females liked big heads then males will develop big heads, no matter how useless they may be for anything else (look at Asian elephants for example as Darren recently covered on Tet Zoo). Do not underestimate that power of selection to be really odd, though personally I do think they were likely fighting in some way, but I also think display plays a part in this. I have a paper in review that deals partly with this, so I’ll keep quiet on the details for now if that’s OK.

At least for Pachycephalosaurus, I’ve always liked the flank butting hypothesis–mostly just because of the tiny impact surface of the skull. From what I’ve seen of the post-cranial skeletal (admittedly in photos–haven’t had a chance to head to Colorado), its neck doesn’t lend itself to a harsh direct impact too well either.

The pachycephalosaurs definitely need more love in the popular press! I got to hold a piece of the head dome last year, and to my amateur hands it seemed like the original bone would have been really dense! I was quite impressed.

I, too, find it hard to believe that an animal with such an impressive noggin wouldn’t find a use for it – even if all the she-pachys just liked men with thick skulls, wouldn’t those boneheads find some use for their natural endowments? Isn’t that what evolution is all about – co-opting existing structures for novel purposes?

PS: Will said paper be available for enthusiasts like myself to download and read?

All my papers are accessible in the you just have to ask and I’ll send out a copy of anyhting requested. But it has onlyjust gone for review so don’t expect it to be coming out for, well, probably anyhting up to 18 months at least.

And yes a lot of evolution is about co-opting structures, but that does not mean they always *are* co-opted. You have to show that rather than assume it and really there is noting quite like these domes out there to act as an analogue (though some artiodactyls and some teleosts come close).

Pachycephalosaurs definately don’t get the coverage they deserve, although the scarcity of good material is partially to blame. Have you seen Sereno’s new tiny bonehead, Mycocephalae? I don’t think it’s been formally announced, but HE announced it in a British video about pachycephalosaurs…you can see it here, at Ville’s blog.

If the ‘your’ is aimed at me then yes, it was the mutual sexual selection paper which did indeed (if very briefly) cover pachycepahlosaurs but obviously far more generally dealt with issues of selection and mate choice and functionality of such structures.